The present invention relates to over-the-horizon communications systems. More specifically, but without limitation thereto, the present invention relates to transmitting and receiving over-the-horizon communications signals using atmospheric and other means to scatter optical radiation from a laser.
Reliable communications systems with high data rates are important for both military and commercial activities. The high bandwidth of optical devices and fiber optics has led to replacing traditional copper wire transmission links with fiber optic cables driven by optoelectronic devices. The optical fiber acts as a waveguide to confine modulated optical energy to a core region over large distances with limited attenuation.
Optical communications systems are also advantageous for secure communications because the signal escaping the fiber optic cable is difficult to detect and is also difficult to interfere or jam. However, fiber optic cables are not practical to use in many applications where one or more of the communications terminals is moving or located in an environment unsuited to fiber optic cables. Communications satellites may be used effectively for optical communications due to the low attenuation of light transmissions in free space. Terrestrial terminals may be located in a dry environment such as a desert or at elevations above cloud level to avoid scattering due to clouds and fog.
Free space communications generally requires line-of-sight, or a direct optical path from the transmitter to the receiver. Communications at sea level prohibit line-of-sight communications beyond distances of about ten miles, limiting the range of optical communications for ship-to-ship use.
Radio waves may be used for over-the-horizon and other environments that do not afford line-of-sight between communications terminals. A disadvantage of radio waves is the relatively low bandwidth available, even with UHF satellites. The competition for bandwidth makes it difficult to gain access to communications channels.
Another approach to non-line-of-sight communications is described in U.S. Pat. No. 4,493,114 by Geller, incorporated herein by reference thereto. Geller describes using an ultraviolet lamp as an omnidirectional transmitting source for short range communications. At longer ranges, however, ultraviolet wavelengths are severely attenuated by the atmosphere.
The pulse position modulation (PPM) described by Geller is widely used for high data rate optical communications. In this technique each character in the sender's alphabet is represented by a binary code. The terms "alphabet" and "character" are used to refer generally to all symbols that may be communicated in messages. These symbols include but are not restricted to alphabetical and numerical symbols. The binary codes are transmitted as a series of optical pulses in consecutive time blocks each having a number of time slots N. Each time block represents a character determined by timing the optical pulse to coincide with a corresponding time slot q in the range of the N time slots.
For example, if N=256, which is equivalent to eight bits, then each time block would be divided into 256 time slots. A pulse having a duration of no more than the time block interval divided by 256 is timed to coincide with a time slot q within the time block. A single detected photon could theoretically communicate 256 bits of information using this technique. More typically, however, one or two bits are communicated by each photon. At one bit per photon, a 10 megabit data rate requires 10.sup.7 photons per second. For an optical beam having a wavelength of about 1.06 .mu.m at a power level of one watt, approximately 5.times.10.sup.18 photons per second may be generated. It is therefore possible to achieve high data rates with low power optical sources assuming that no stray photons of the same wavelength enter the detector. Unfortunately, preventing stray photons from reaching the detector is difficult, resulting in a bit error rate when more than one time slot in a time block contains a detected photon.
The PPM format for optical communications is appropriate in applications where the solar background may be eliminated or when the signal intensity is substantially higher than the background intensity. In line-of-sight optical communications systems, the field of view of the detector may be made narrow enough so that only the emission of the optical transmitter is received. For other than line-of-sight applications, the bit error rate of the PPM format becomes a problem. Each time more than one time slot in a time block of N time slots contains a detected photon, or if no photon is detected in any time slot, N bits of data are lost. Synchronization of the transmitter and the receiver also becomes more difficult in non-line-of-sight applications, where only a small fraction of the transmitted power reaches the detector. Typical causes of signal losses are low scattering probability in the atmosphere and divergence of the transmitted beam.
The intensity of scattered light relative to beam intensity may readily be demonstrated by observing a blue-green laser in a dark room. Low levels of scattered light from particulate and molecular scattering are readily visible to an observer located to the side of the beam propagation path. If a white card is placed in the beam path, the intensity of the beam may be visually compared to the atmospheric scattering. The difference is a dramatic illustration of how small a fraction of optical power is scattered normal to the propagation path. While low scattering environments propagate only a small fraction of the transmitted power, high scattering environments such as smoke and clouds severely attenuate the transmitted beam, reducing the effective range.
A continuing need therefore exists for an optical communications technique that is suitable for over-the-horizon communications and has a low bit-error rate.